(1) Field of the Invention
This invention relates generally to switched-mode amplifiers (Class-D amplifiers) and relates more specifically to switched-mode amplifiers having reduced distortion.
(2) Description of the Prior Art
Switched-mode amplifiers are often referred to “Class-D” amplifiers. Output of these types of amplifiers is a pulse-width (PWM) signal or a sigma-delta bit stream. Before applying such signal to a speaker load it is often filtered to remove or attenuate undesired high-frequency carrier frequency. Recently, filter-less switched systems have become popular. In filter-less architecture output of a switched-mode amplifier is often directly connected to a load.
Typical prior-art switched-mode power amplifier architecture of an audio application is illustrated in FIG. 1 prior art (output filter is not shown). FIG. 1 prior art demonstrates that a typical switched-mode amplifier consists of: a loop filter 1, a feedback/gain block 2, a pulse-width modulation (PWM) block 3, an oscillator 4 (triangular wave generator), output drivers 5, and a power stage 6. The loop filter 1 integrates, e.g. audio signals and high-frequency pulse-width modulated output signals. In addition, the loop filter 1 is also required to attenuate residual high-frequency signals. The PWM block 3 converts the output signals from the loop filter 1 into pulse-width modulated (PWM) signals by comparing the loop filter output signals with the triangular wave signals from the oscillator 4. The output signals of the PWM block 3 are being applied to the output driver block 5 and to the power stage 6. The output driver 5 and power stage 6 blocks are required to provide a most efficient energy transfer from power source to the speaker load. Usually an output filter is used between the power stage 6 and an audio load. Typical efficiency requirement of these blocks is above 80%.
Such high efficiency is achieved by using output devices of the power stage 6 in switched-mode (full ON-OFF). The audio performance of such a switched mode system can be affected by distortion and noise performance of the output stage (power supply noise, switching artifacts; “dead time”, finite rise/fall edges, etc). Therefore global negative feedback by feedback/gain block 2 is typically applied around the entire amplifier in order to minimize these unwanted effects.
An input loop filter is typically required to provide high open loop gain and attenuation of PWM carrier frequency. A high in-band open loop gain leads to improved distortion, noise and power supply rejection performance. High attenuation of the PWM carrier frequency is also desirable since residual high-frequency components (such as 2×f clock due to double transition at each switching period) present at the output of the loop filter can cause intermodulation (aliasing) with triangular wave signal/fclk) and input audio signal. These unwanted intermodulation products are signal-dependent and can fold back into base-band and form harmonics of the input audio signal. These harmonics become distortion products of a switched-mode amplifier. Due to such intermodulation distortion performance of conventional switched mode amplifier is often limited by finite attenuation of residual carrier signals. Conventional switched-mode amplifier topologies require trade-off between high in-band loop gain and high attenuation of the clock carrier frequency. Typical prior art architectures exhibit limited distortion performance. High-order filters can be employed in order to provide additional attenuation of the carrier signal. However such approach often leads to increased complexity and often to reduction of stability margins of the system.
It is a challenge for the designers of switched-mode (class-D) amplifiers to design circuits wherein improved harmonic distortion, high attenuation of residual unwanted ripple signals without an undue reduction of stability margins and in-band open loop gain are achieved.
There are known patents or patent publications dealing with switched-mode amplifiers:
(U.S. Pat. No. 7,471,144 to Lipcsei) proposes a low distortion class-D amplifier comprising a power supply terminal for receiving power, an amplification stage, and an output stage. The amplification stage receives an input signal and generates a ramp signal.
(U.S. Pat. No. 7,301,393 to Candy) discloses an electronic class-D amplifier with intrinsically low distortion employing modulation of the slopes of a triangular-wave carrier reference signal. The amplifier includes a pulse width modulator, which includes an oscillator.
U.S. Patent Publication (US 2007/0132509 to Mochzuki) describes a class D amplifier that is capable of reducing distortion of a specific sampling frequency, and frequencies that are multiples of this frequency to a level where an LPF is not required and small-scale control circuit.
Furthermore (U.S. Pat. No. 7,002,406 to Risbo et al.) discloses a class-D amplifier circuit providing improved open-loop error for base-band frequencies, such as the audio band. The amplifier circuit includes a comparator for generating a pulse-width-modulated output signal that is applied to an output power stage. An LC filter is at the output of the power stage.
(U.S. Pat. No. 7,227,408 to Candy) teaches an electronic class-D amplifier including a pulse width modulator and a servo-loop amplifier characterized in that there is provided a sample and hold element which samples a signal at the sample input when a sample command signal is applied to a control input, and holds the value of the last sampled value of the signal at the sample input when a hold command signal is applied to the control input.